IR optical imaging systems are well-known, comprising camera-like elements in which incident IR radiation is focused on a detector or a plurality of detectors which generate electrical signals varying in accordance with the intensity of the radiation focused upon the detector's surfaces. An image of the incident IR is often focused on a two-dimensional array of detector elements in the same manner as radiation in the visible spectrum is focused on the plate or film plane of a conventional camera. However, since the output of the IR detectors is an electrical signal, some systems use only a single line of detector elements in combination with scanning mirror arrangements, and the image of the incident radiation is broken up into a series of linear components which are sequentially scanned across this single line of detectors, the output of each scan being stored and thereafter used to recreate the overall IR pattern electronically.
The lens elements of these IR optical imaging systems are manufactured from crystalline semiconductive materials such as Ge, Si, ZnSe, CdTe, GaAs, which have a refractive index with high temperature coefficient. That is, these materials are very sensitive to temperature variations and, as a result of this, the focusing of an IR optical imaging system is to a great extent a function of temperature.
To eliminate the effect of temperature changes on the focal length, i.e., the focus, of an IR optical imaging system, the prior art has suggested encasing the entire IR imaging system in a temperature-controlled housing, or adjusting of the imaging lens elements to predetermined focusing positions which are selected according to ambient temperatures. However, these solutions add significant complexity and expense to the manufacture and use of the lens systems.
The prior art also shows (in U.S. Pat. No. 4,148,548) an IR imaging system in which one or more of the optical lens elements are movable relative to each other and in which relatively complex iterative procedures are suggested for adjusting the position of these movable lenses to achieve optimum focus under various temperature conditions.
Such known temperature compensation systems are all relatively complex and, perhaps more importantly, do not take into account that radial temperature gradients occur in addition to axial ones in the IR optical system. Therefore, these prior art systems provide temperature compensation that is less than satisfactory in many practical applications.
The invention disclosed herein solves the problems referred to above in a relatively simple manner with a method and apparatus, suitable for automation, which compensates for radial as well as axial temperature gradients in the lenses of IR optical systems.